It happens to all of us: we stay out too late or we wake up in the middle of the night fretting about some thing or another and then we end up sleep deprived, and sure enough a few days later we come down with a cold or the flu. It's the reason our mothers used to hate letting us have sleepovers. It's why we spent so much of college feeling like shit. And it's probably the reason why those of us with young children catch every bug they bring home from school. But even though we intuitively understand that rest is important to our immune system, scientists are only just starting to figure out exactly why that is, and the answer is actually pretty cool.

The key to the whole thing lies with our body's innate circadian rhythm, which operates on a 24-hour cycle and controls various mental and physical processes. Our brains control the rhythm, and our behavior can affect it. So if we get jet-lagged, for instance, it's because we've thrown our circadian rhythm off.

Besides making us feel normal or like zombies, depending on how good our rhythm is at the moment, it also has effects on the cellular level. To figure out how this relates specifically to our immune function, researchers looked at a protein known as the "toll-like receptor 9" that is a key part of the immune system. It works to detect infections so your immune function can amp up and fight them off. The researchers wanted to find out whether that protein was under circadian control. To do that, they measured the level of the protein in mice and found that it varied throughout the day with the circadian rhythm. Researchers also discovered that if they gave the mice vaccines at the time when the protein was at its peak, immune response was improved.

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Yale University Professor Erol Fikrig, who led the study, said this indicates a "direct molecular link between circadian rhythms and the immune system." This means that any changes to that rhythm have consequences. According to him, "It does appear that disruptions of the circadian clock influence our susceptibility to pathogens." So in other words, when we don't get enough sleep or screw up our sleep schedule, we're more likely to get sick.

Beyond making us feel smart for having already observed this pattern ourselves, having a scientific understanding of why is important for a number of reasons. For one thing, after further research has been done, it could dramatically change the way that we treat infections. We might give drugs and vaccines at certain times to boost their effectiveness, or drugs might be developed that change the body clock and put the immune system into its most powerful phase.

That's all a ways off, of course, though Dr. Akhilesh Reddy, a circadian rhythms researcher at the University of Cambridge, said drug companies are actively researching this topic, and he expects to see the body clock playing a role in medicine, "within 10 years." Bring it on. In the meantime, the best thing to take from this finding is that if you're trying to stay healthy, try to maintain your circadian rhythm with as little disruption as possible. That means no screwing up your sleep schedule, which rules out all kinds of fun, and keeping to a steady routine of eating and exercising too. Can you hear a gentle chorus of mothers sweetly saying, "I told you so"?